This invention relates to reducing pollutant concentration in a process gas stream. More particularly, this invention oxidizes soot and products of incomplete combustion in internal combustion engine exhaust emissions by use of a flameless thermal oxidizer.
Internal combustion engines produce emissions containing water vapor, products of incomplete combustion such as, carbon monoxide and unburned hydrocarbons, carbon dioxide, oxides of nitrogen [NOx], carbonaceous soot and other combustible particulate matter, and other particulates and gaseous constituents. Oxides of nitrogen, products of incomplete combustion, and particulates are considered atmospheric pollutants. The particulate matter may also contain condensed hazardous compounds
Such emissions produce well-known harmful effects to environmental quality and human health. For example, engine soot emissions contribute to reduced atmospheric visibility and particulate fall out, and have been found to contain carcinogenic polycyclic aromatic hydrocarbons, such as naphthalene, acenaphthylene, anthracene, and chrysene. B. S. Haynes and H. G. Wagner, Soot Formation, Progress in Energy and Combustion Science, Vol. 7, at p. 229 (1990).
Further, because of its particle size, the particulate matter from diesel exhaust represents a respiratory health hazard. The particle size distribution of particulate matter from diesel engine exhaust is typically 80% minus 10 microns, and 77% minus one micron, based on aerodynamic particle diameter.
In response to air quality regulations, vehicle manufacturers install pollution control devices-in internal combustion engine exhaust systems. Traditional engine pollution control devices employ a ceramic honeycomb monolith or a packed bed of pellets having a coating of a noble metal catalyst. Such devices catalyze the reactions of carbon monoxide and unburned hydrocarbons with oxygen, typically at approximately 500xc2x0 F. to 800xc2x0 F. Other devices employ catalysts that also catalyze the reaction of oxides of nitrogen. Unfortunately, two factors render such catalytic devices unsuitable for soot-laden gases that are commonly produced by diesel engines. First, the catalytic devices are ineffective at destroying soot. Second, the soot and other particulates deposit on the monolith, thereby preventing gaseous constituents from reaching the catalytic material, or possibly deactivating or poisoning the catalyst. Further, spent catalyst also may be classified as a hazardous substance. Moreover, such devices induce a substantial back-pressure on the engine, which reduces engine efficiency. Further, sulfur that is found in diesel and gasoline fuels can poison or deactivate the catalyst.
A technically feasible method of reducing soot emissions is to pass engine exhaust gas through a ceramic filter that can periodically be replaced or regenerated. These filters, however, have only 85% removal efficiency, impose a significant back pressure on the engine, and are expensive. Filter manufacturers estimate that filter prices would drop no lower than U.S. $8,800 each, even with economies of scale because of increased production. Control of Air Pollution From New Motor Vehicles and New Motor Engines, Federal Register, Vol. 58, No. 55, Mar. 24, 1993, p. 15786 (1993). Furthermore, the engine back pressure caused by the ceramic filter adds U.S. $2,000 in annual fuel costs to a typical urban bus because of reduced engine efficiency. Id.
One type of filter trap design regenerates itself by burning some engine fuel periodically, thereby oxidizing the soot accumulated on the filter surface. Another trap design continuously regenerates with the use of a catalyst. The latter trap design has achieved reduction efficiency of between 80 and 92% for particulate matter. Focus on Industry Solutions for Exhaust Pollution Control, Automotive Engineer, Oct./Nov. 1994, at p. 18. Unfortunately, regenerative trap features add even more to the filter cost.
Thus, no commercially viable method currently exists for removing soot and other particulates from engine exhaust gases. The lack of effective soot treatment methods is especially problematic for diesel engines that produce high soot emissions. Despite the difficulty in controlling such emissions, the U.S. Environmental Protection Agency (xe2x80x9cEPAxe2x80x9d) has implemented regulations restricting particulate matter emissions from buses and other heavy duty engines. Control of Air Pollution From New Motor Vehicles and New Motor Engines, Federal Register, Vol. 58, No. 55, Mar. 24, 1993, p. 15781.
Although eliminating particulate matter from diesel engines has been an intractable problem, industrial gas cleaning techniques have been employed to collect particulates in other applications. One technique for collecting and removing particles from a gas stream is electrostatic precipitation, which uses electrostatically charged surfaces to collect charged particles. An electrostatic precipitator device (xe2x80x9cESPxe2x80x9d) imparts a charge on particles within a gas stream by exposing the particles to an electric field. Plates or cylinders, which have a charge opposite that of both the electric field and the particles, attract and collect the charged particles. Conventional ESPs intermittently clear collected particles from the collection surface. Conventional dry-process ESPs clear collected particles by mechanical methods, such as mechanical shock or rapping, and conventional wet-process ESPs flush the particles with a liquid. After the particles are cleared from the collection surface, the particles fall into a hopper for disposal. Conventional ESPs are limited by the temperature limits of the internal components, and flammable gas constituents entering a conventional ESP are controlled to avoid ignition by arcing within the ESP.
Another technique for collecting particles entrained within a gas stream is centrifugal separation using a cyclone. In a conventional cyclone, an inlet air stream is directed to form a vortex. Centrifugal forces push particles within the gas stream to the wall of the cyclone shell, where they lose momentum and fall out of entrainment. Because the collection efficiency of a certain particle size depends on the mass and aerodynamic diameter of the particle, cyclones have higher collection efficiency on larger, more massive particles. For example, cyclones are generally effective at removing particles of greater than about three to five microns. Neither a conventional cyclone nor a conventional electrostatic precipitator can effectively reduce the large component of the diesel exhaust particulate matter that has a diameter of less than one micron.
In addition to regulations governing particulate matter and hydrocarbon emissions, internal combustion engines are the subject of regulations limiting NOx emissions. Oil and Gas Journal, Jul. 25, 1994, p.42. The simultaneous emission limits for both particulate matter and NOx presents a unique problem because the two pollutants typically have an inverse relationship in engine exhaust. Internal combustion engines generally can be configured and tuned to produce emissions having low soot and high NOx concentrations or, alternatively, high soot and low NOx concentrations. Traditionally, engines that employ catalytic devices are adjusted to minimize soot formation because of the catalysts"" inability to handle high temperatures inherent in combustion of soot. Tradeoffs also typically compromise engine efficiency. Such adjustments result in high levels of NOx emissions.
In efforts to comply with regulatory limits, diesel engines have been redesigned to reduce particulate emissions. Such redesigns include, for example, a dramatically different combustion chamber design, manufacturing the engine with tighter bore tolerances to reduce the introduction of oil into the combustion chamber, and increasing injection pressures. Magdi K. Kahair and Bruce B. Bykowski, Design and Development of Catalytic Converters for Diesels, SAE paper 921677, p. 199. Although helpful, such redesigns have been inadequate to meet present and contemplated future regulatory emission limits. Advanced common rail high pressure injection of fuel is the primary technology for reducing particulate mass.
Although not generally employed in reducing NOx emissions from internal combustion engines, various techniques exist for reducing NOx emissions from gas streams in other applications. One technique for reducing NOx emissions is selective catalytic reduction (SCR), which destroys NOx in the presence of ammonia (NH3) over a catalyst. Although selective catalytic NOx reduction is capable of high levels of NOx removal, the temperature of the exhaust must be in the range of 550xc2x0 F.-800xc2x0 F., which is typically below internal combustion engine outlet temperatures. Furthermore, the catalyst has the limitations discussed hereinabove.
Another approach for removing NOx is selective non-catalytic reduction (SNCR), which employs a chemical that selectively reacts, in the gas phase, with NOx in the presence of oxygen at a temperature greater than 1150xc2x0 F. Chemical NOx reduction agents used in such processes include ammonia (NH3), urea (NH2CONH2), cyanuric acid (HNCO)3, iso-cyanate, hydrazene, ammonium sulfate, atomic nitrogen, melarnine, methyl amines, and bi-urates.
Additional recent regulations require automobile manufacturers to reduce emission of organic vapor from vehicle fuel tanks. Typically, fuel tank control devices have a layer of activated carbon that absorbs the vapors and prevents their escape to the atmosphere. Periodically, the control devices require regeneration or replacement with fresh adsorbent material. Unfortunately, these devices are complex and expensive. Id. Another option for reducing fuel tank emissions is to process them through the existing catalytic device in the engine exhaust system However, conventional catalytic devices are generally unsuitable for use with concentrated fuel tank vapor. Specifically, concentrated fuel vapor combustion may raise the monolith temperature above the catalyst""s upper temperature limit, thereby thermally deactivating the catalyst.
Therefore, it is an object of the present invention to provide a system and method for reducing internal combustion engine pollutant emissions in response to regulatory emission limits. Specifically, an object of the present invention is to provide a system and method for reducing soot concentration in an engine exhaust stream that overcomes the limitations of the prior art.
It is another object of the invention to provide a system and method for reducing soot concentration, while simultaneously enabling the reduction of NOx concentration, in an engine exhaust steam.
It is yet another object of the invention to provide a system and method for reducing the fuel vapor emissions from an engine fuel storage tank.
In order to achieve the above and other objects of the invention, a system and method for establishing reaction of an internal combustion engine exhaust stream within a flameless thermal oxidizer are provided. Upon initiation of the reaction, pollutants contained within the exhaust stream, especially products of incomplete combustion and soot, react within a self-sustaining reaction wave. The flameless thermal oxidizer comprises a matrix of heat-resistant media in which the reaction wave forms. Additionally, an air stream and a supplemental fuel stream may be provided to supply reactants, and a hot gas stream may be provided to supply process heat. Fuel tank vapors are included in the supplemental fuel stream.
Oxidizing soot within inert media is effective for several reasons. Uniform flow promotes even particle burnout and efficient use of space. Also, soot particles having high momentum may be captured within the matrix by inertial impaction. Because the media is inert, capturing enhances burnout of the soot, as distinguished from catalytic devices in which capturing poisons or deactivates the catalyst. A flameless thermal oxidizer, therefore, is capable of removing and destroying a greater portion of soot in an internal combustion engine exhaust stream (compared with filter and catalytic systems). Destruction and removal efficiency (xe2x80x9cDRExe2x80x9d) for soot in a flameless thermal oxidizer according to the present invention is between 88% and 97% for vehicle engines and 99.99% for stationary engines, depending on the particular configuration and soot residence time. Further, the thermal oxidizer may include means to increase the residence time of particles. Even high soot DRE may be achieved by optimizing the thermal oxidizer size. Moreover, a flameless thermal oxidizer""s inherent heat recuperation enhances thermal efficiency.
The system according to the present invention comprises an internal combustion engine for producing an exhaust stream and a flameless thermal oxidizer containing a matrix of heat-resistant media The flameless thermal oxidizer includes an inlet plenum for premixing and distributing the flow, a shell for housing the matrix, a heater for preheating the matrix and/or initiating the reaction, and a control system. The control system controls and adjusts the reaction wave by modifying the flow rate of the engine exhaust stream, air stream, supplemental fuel stream, and/or hot gas stream, or by controlling the heater. The position of the reaction wave is ascertained by plural temperature sensors disposed within the matrix along the flow path of the process stream.
The thermal oxidizer described herein may be located downstream from an engine, and between an engine exhaust and a turbo-charger. The latter location diminishes turbo-charger wear due to particle erosion and promotes thermal efficiency in at least two ways. First, ambient heat loss from the engine exhaust stream is minimized by close spacing between the components. Second, the increased enthalpy of the exhaust stream may be recovered by expansion through the turbine of the turbo-charger, which would reduce the overall energy of the added fuel (if any).
The flameless thermal oxidizer according to the present invention comprises three main embodiments, in addition to the embodiments employing means to improve particle retention time. First, in its conceptually simplest form, the process stream flows into one end of a cylindrical matrix and exits the opposing end. Second, the flameless thermal oxidizer may also have plural feed tubes that extend longitudinally through a cylindrical matrix. The process stream flows through the feed tubes and then through the matrix counter-current from the flow within the feed tubes. Because the exothermic reactions occur within the matrix, the process stream within the matrix transfers heat to the incoming stream within the feed tubes. A plenum may be located at the distal end of the matrix for introducing an air stream, a supplemental fuel stream, and/or a hot process gas stream for preheating purposes. Third, the flameless thermal oxidizer may have a single center tube that extends to the end of the matrix. The process stream flows longitudinally through the center tube, and flows radially from the center tube through tube ports, the matrix, and through shell holes.
In addition to the substantively planar reaction waves (occurring in the first two embodiments) and substantively cylindrical reaction waves (occurring in the third embodiment), the method according to the present invention encompasses forming reaction waves in a Bunsen form, a Burke-Schumann form, and an inverted V shape. The thermal oxidizer may employ an engineered matrix to form a reaction zone of these or other shapes.
In addition to destroying soot and products of incomplete combustion from internal combustion engines, the present invention also encompasses a system and method for reducing emission of oxides of nitrogen. Because of the inverse relationship between the formation of soot and NOx in internal combustion engines, the engine may be adjusted to produce a minimum NOx concentration and a high concentration of soot. Upon reduction of the soot content of the engine exhaust stream by a flameless thermal oxidizer, the resulting exhaust system is low in NOx concentration because of the engine adjustments and because the flameless thermal oxidizer produces a minimal amount of thermal NOx. Further, a catalytic device for removing NOx may now be effectively employed downstream of a flameless thermal oxidizer since the flameless thermal oxidizer will destroy soot that might otherwise poison or plug the catalyst.
In another aspect of the present invention, a system and method for the simultaneous destruction of soot and NOx employs a thermal oxidizer into which a reductant stream is injected. The reductant destroys NOx according to, preferably, the selective non-catalytic reduction technique, although the present invention encompasses other NOx reduction techniques, such as SCR.
Because oxidation of a combustible particle depends on both exposure to high temperature and the time period of such exposure, increasing the residence time of the particles within the thermal oxidizer improves destruction and removal efficiency (xe2x80x9cDRExe2x80x9d) for particles. Specifically, a particle of a certain large aerodynamic size and mass may theoretically pass through a conventional thermal oxidizer before complete oxidation of the particle, depending on process equipment parameters.
To increase destruction and removal efficiency of soot, the flameless thermal oxidizer may be sized to provide a sufficient residence time to destroy the largest statistically relevant particle size, but such large sizing increases costs and uses increased space. To increase the destruction and removal efficiency (xe2x80x9cDRExe2x80x9d) of particles for a given thermal oxidizer size, an aspect of the present invention employs techniques to increase selectively the residence time within the thermal oxidizer of particles. These techniques include equipment and methods for using electrostatic forces and centrifugal forces to attract and temporarily to collect or diminish the effective velocity of the particles.
The electrostatic technique comprises imparting a charge onto particles and attracting and collecting particles within the thermal oxidizer. The charge is imparted by passing the particles through an electric field created by the electrical corona of a discharge electrode. The process stream is directed through a collection tube that is electrostatically charged with a polarity, preferably positive, that is opposite that of the discharge electrode. Because the particles have a polarity opposite that of the collection tube, the particles flow across the gas streamlines to the collection tube, where the particles reside until oxidized.
The centrifugal force technique comprises imparting an angular velocity component or spin onto the process stream as it longitudinally flows through a center feed tube. Particles that reach the surface of the tube either adhere to the wall or lose velocity because of boundary layer effects. Particles that adhere to the wall oxidize, while those that lose velocity have increased residence time.
Other and further objects and advantages will appear hereinafter.